This invention relates generally to improvements in the laminated boards used in electronic circuitry.
Printed wiring boards (PWB's) are composites of an insulating core and one or more planar layers of circuitry. The insulating core comprises a thermosetting polymer, such as an epoxy resin, and a suitable reinforcing material, such as glass cloth. The core may be faced with a layer of copper foil on one or both sides to provide an electrically conductive path. The circuit patterns are created by photoimaging and etching of unwanted copper from these layers. Where there are multiple layers, the circuits are interconnected by through-plated holes precisely positioned in predetermined locations.
A number of patents have suggested the use of a strippable aluminum sheet as a protection and support for very thin layers of copper such as those which result from vapor deposition or electroplating processes. The layers are so thin, say less than 5 .mu.m, that they are impractical to handle alone and a support is needed. One example may be found in U.S. Pat. No. 3,990,926 where copper is electroplated onto a disposable base sheet of aluminum, zinc or steel. An intermediate layer of a metal such as zinc or tin was suggested to improve adhesion.
In U.S. Pat. No. 4,394,419 an ultra-thin layer (1-12 .mu.m) of copper was plated onto a copper carrier sheet after electroplating a thin intermediate layer of nickel, nickel-tin alloy, nickel-iron alloy, lead, or tin-lead alloy so that there are two distinct layers of copper. The present invention avoids the need for successive electroplating of metal layers on a copper foil and is concerned with more conventional weight copper foils of a thickness above about 8-12 .mu.m.
In U.S. Pat. No. 5,153,050 another problem is discussed, namely, how to avoid damage or contamination of the thin copper foils disposed on the outside of multilayer laminates. The patentees proposed gluing a temporary layer of aluminum sheet onto copper foil before it is bonded to an insulating core by heating under pressure to cure the polymers in the core.
The general concept of gluing a protective sheet to copper foil is found also in U.S. Pat. No. 5,120,590 where, rather than aluminum, a sheet of a suitable plastic film was used to protect the copper foil until after it has been laminated to the core.
While the use of aluminum sheets to protect multilayer laminates has merit, it is evident that the patentees did not recognize the thermal expansion problem addressed in U.S. Pat. No. 5,350,621, incorporated by reference herein. Aluminum sheets also will interact by friction with the copper foil, which could result in damage to the foil and distortion resulting from the difference in the coefficient of expansion of aluminum relative to copper (aluminum expands about 1.5 times as much as copper). Also, drilling through an aluminum sheet may produce undesirable contamination of the copper circuit patterns.
The present inventors propose an improved method of protecting multilayer laminates which incorporates the insights of U.S. Pat. No. 5,350,621 and uses a sheet copper or a metal having a similar coefficient of thermal expansion to protect multilayer laminates during processing.